The steam generators of pressurized-water nuclear reactors usually comprise a bundle consisting of a very large number of tubes of small diameter, inside which circulates, at a high temperature, the pressurized water coming from the reactor vessel.
The feed water of the steam generator comes into contact with the outer surface of these tubes, as a result of which the feed water is heated and evaporated by the heat of the pressurized water constituting the heat-exchange fluid of the reactor. The tubes of the bundle are bent in the form of a U and, at each of their ends, are fastened in a tube plate fixed to the casing of the steam generator. The pressurized water is distributed and recovered in the tubes by a generally hemispherical two-part water box located under the tube plate.
The many stresses to which the tubes of the steam generator are subjected, for example the phenomena of stress corrosion, can cause damage to the tubes. When a tube is damaged, it has to be put out of action quickly, in order to prevent the primary heat-exchange fluid containing radioactive elements from escaping from this tube and mixing with the feed water constituting the secondary fluid some of which is discharged into the atmosphere in the form of steam. Defective tubes are put out of action by plugging their two ends in the region of the tube plate.
To carry out this plugging operation, it has been proposed to use a manual appliance ensuring that the plug is introduced into the tube and expanded inside the tube. A plug consisting of a cylindrical piece of a diameter slightly less than the inside diameter of the tube and having an inner bore is screwed to the end of the appliance which is subsequently introduced into the tube to be plugged. The control of the appliance makes it possible by means of a jack to exert a pull on a rod, ensuring the diametrical expansion of the plug inside the tube.
The disadvantage of such a process is that an operator needs to be present inside the water box to operate the manual appliance. This operator is therefore exposed to a radioactive environment, and consequently the time spent on his work must be reduced to a minimum. The material and psychological conditions of this work are also unfavorable, and because of this the operator, when he works for the maximum time allowed, can only do a very incomplete job.
In the maintenance and repair of a steam generator which has several defective tubes, it is necessary to provide a relatively large team of qualified trained operators.
To overcome these disadvantages and make the job of maintenance operators easier, automatic machines making it possible to plug the tubes by remote control have been developed, the operation being controlled and monitored from a a work station located outside the water box. Such machines, comprising a device for supporting, moving and positioning the plugging appliance, generally have a magazine in which the plugs are stored and which has a form similar to that of a loader for automatic weapons. However, such a magazine has a limited capacity, as a result of which the period of operation of the machine inside the water box is restricted.